On Sunday, Carter, 91, told a church group in Georgia that his physicians could not find any cancer in his scans. “My most recent MRI brain scan did not reveal any signs of the original cancer spots nor any new ones,” he said in a statement. Carter says he will continue taking his regular 3-week immunotherapy treatments of the cancer drug pembrolizuma, which has shown promise in the treatment of melanoma.

So what exactly does it mean to be “cancer-free”?

Not seeing any cancer on imaging tests is promising, but a physician cannot be 100% certain that means a cancer will not return. Even with potent treatments there’s a chance that cancer cells could survive and grow over time. As the American Cancer Society notes, this is why doctors will rarely say a person is “cured.” Instead, they may say things like, “The cancer can’t be seen on the scan” or “I see no evidence of any cancer.” Usually by two or three months of treatment a doctor can look to see how a person’s cancer is responding.

Not all cancers will recur, and if cancer cells do survive treatment it could take years before they develop into identifiable disease.

According to the National Institutes of Health (NIH), if someone remains in complete remission—meaning all signs and symptoms of cancer are gone—for five or more years, some doctors may say a person has been “cured.” Still, cancer cells could remain in the body for many years after treatment.

Carter started his treatment in August, which consisted of a combination of radiation and a newer drug. The drug he was given, pembrolizumab (brand name Keytruda), is part of a rapidly growing class of drugs called immunotherapy, which uses the body’s immune system to fight the cancer. The drug received accelerated approval by the U.S. Food and Drug Administration (FDA) for certain conditions in 2014. In general, immunotherapy works in different ways from chemotherapy. Some treatments work by enhancing the immune system overall and other treatments specifically target cancer cells.

Using the immune system as a weapon against cancer makes a lot of sense. The job of the immune system is to identify “foreign” substances in the body and attack them. This is how our body kicks a cold or the flu. However, the immune system has a harder time identifying cancer cells as foreign, sometimes due to the fact that they don’t appear different enough from normal cells or the immune system isn’t powerful enough to take on the cancer. By using different strategies to bolster the immune system, researchers can get the immune system to better recognize cancer cells and launch a stronger attack. Some researchers are also looking into how to harness the knowledge gained from immunotherapy to develop cancer vaccines.

Immunotherapy has proven effective for melanoma, the type of cancer for which Carter received treatment. Reuters reports that about 30% of people treated with pembrolizumab experience significant tumor shrinkage. About 5% go into complete remission.

For now, President Carter will continue part of his treatment as he announced, and his status will likely be regularly monitored.